Heating and Air Conditioning (HVAC) systems for modern buildings and factories are generally precisely regulated to control the amount of outside air introduced into the system. In such systems, the designer must balance the need for energy conservation, which entails minimizing the amount of new outside air which must be introduced, and therefore heated or cooled, vs. the competing need for adequate fresh air ventilation to prevent the accumulation of stale air and the accompanying effects of so-called "sick building syndrome" on occupants.
Typically, in such controlled HVAC systems, outside air is introduced via selectively controllable dampers. For example, a damper can be a rectangular frame built into a wall communicating with the exterior of the building. Within the rectangular frame, a plurality of rotatable vanes are positioned, which vanes are selectively rotatable between a vertically oriented, completely closed position at which no air is introduced, and a substantially horizontally oriented, completely open position at which maximum air is introduced. Between these extreme positions are an infinite number of intermediate, partially open positions.
It is also common to associate a moisture and particle elimination louver with the controllable damper at the air inlet to an HVAC system. Conventional moisture elimination louvers have a disadvantage of presenting a substantial resistance to air flow and thus significantly lowering the potential air velocity through the louver. For example, a typical moisture elimination louver will restrict air flow to a maximum of 500 FPM face velocity. Minimum velocities are typically about 20% of maximum, or 100 FPM.
In order to accurately control the amount of ambient air introduced into a building, the air flow must be measured. The conventional method of sensing air flow is to place a pitot static sensor in the air stream to measure the difference between the upstream and the downstream pressures to determine the differential or velocity pressure. The velocity pressure is proportional to air flow according to the relationship: EQU Velocity Pressure=(Velocity(FPM)/4005).sup.2
However, a practical limit of instrumentation is a velocity pressure of 0.02. With pressures less than this, instrumentation sensitivity does not allow accurate measurement. Therefore, with conventional moisture elimination louvers, with air flow rates of from 100 to 500 FPM face velocity, doubled within the louvers to 200 to 1000 FPM velocity pressures would vary between 0.0025 and 0.0625. Even with air measurement systems which amplify sensed velocity pressure by 3:1, the minimum air flows are well below accurately measurable limits.
It is clear then, that a need exists for an improved system and method for associating differential or velocity air pressure sensors with moisture elimination louvers in a manner such that they can reliably detect velocity pressure at virtually all levels of air flow through the louver.